Core Data: statement is still active

Question

I am getting the following error in my app:

CoreData: error: Serious application error. Exception was caught during Core Data change processing. This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification. statement is still active with userInfo (null)

Everything that I can find seems to indicate the I may be having multi-threading issues with my core data managed object context, but I can't seem to find anywhere in my app where this would be the case. I am accessing and managing a managed object context on a background thread. The context is only fetching and manipulating objects on that one background thread. When I detect saves to that context through NSManagedObjectContextObjectsDidSaveNotification, I am merging the changes into a different context that I only access on my main thread. When I make the call to merge the changes, the error is thrown. It is very rare that this occurs, even with the same data sets.

I read somewhere that it is possible to 'enable multi-threading assertions' using '-com.apple.CoreData.ThreadingDebug 3', but I haven't been able to get this to work. Does anyone know if this is possible? I was hoping this might turn on some assertions that would help me find where I am playing with the context on the wrong thread or something.

Any other clues on what might be happening or how to track this sort of problem down?

Answer 1

I had a similar problem and found a way to solve it.

I've created a mechanism creating different contexts based on thread names (1 thread = 1 context).

#include <pthread.h>
...

    mach_port_t threadID = pthread_mach_thread_np(pthread_self());
    NSString *threadName = [NSString stringWithFormat:@"%x", threadID];
    NSManagedObjectContext *context = [singleton.threadsContexts objectForKey:threadName];
    if (!context) {
        NSLog(@"Creating managed context for thread named '%@'", threadName);
        context = [[NSManagedObjectContext alloc] init];
        [context setPersistentStoreCoordinator:[singleton.managedObjectContext persistentStoreCoordinator]];

        //initialize dictionary in your singleton if it as not been yet
        if(!singleton.threadsContexts)
        {
            singleton.threadsContexts = [NSMutableDictionary new];
        }
        [singleton.threadsContexts setObject:context forKey:threadName];
    }

    return result;

and then, when i need a full version of a managed object in a background thread, I get a copy of the object dedicated to this thread :

NSManagedObjectModel *myNewObject = [myBackgroundContext objectWithID:[myObject objectID]];

Hope this answer will help.